3.10.23 – SSI – Guest Authors
As automatic license plate capture becomes more reliable, affordable and mainstream, myriad applications and integrator opportunities are coming to light.
By Paul Garms
License plate capture and recognition offers many opportunities to enhance security and gather data that extends the applications for these solutions beyond security. For critical sites, monitoring vehicles entering an area and checking their authorization to be there is important for reducing risk. For other organizations, having the capability to capture license plates of vehicles moving in a designated area gives users complete awareness of traffic flow and usage patterns.
There are several components of license plate recognition (LPR) solutions: capturing the plate, managing and acting upon plate information, and integrating the solution into an existing architecture. Each plays a critical role.
Capturing the license plate image with a camera is the first step, but often the cameras do not process the images to retrieve the actual license plate number. For this step, additional LPR software running on a server, appliance, or in the Cloud is needed.
Not all cameras are supported by all LPR software providers, so it is important to verify support before moving forward with a solution. Industry partnerships between hardware and software manufacturers may help to ensure a better-performing solution as well as integration of the LPR solution with other systems in use at the facility.
Capturing the License Plate
As many applications for LPR solutions require operation during the day and at night, it is important that the cameras selected can deliver reliable vehicle identification data in all lighting and weather conditions, even when glaring headlights are present.
Image quality is vital when capturing license plates. Standard cameras are often challenged to capture images and fast-moving objects in changing lighting conditions ranging from bright sunlight to dimly lit or low-light conditions. At the same time, an increasing percentage of noise may occur, leading to grainy images and a loss of detail that may obscure the license plate information.
Dedicated license plate capture cameras or those with a license plate capture scene mode are designed to overcome these issues and provide LPR software packages with the best possible images for decoding the license plate numbers. In addition, next-level low light and high dynamic range (HDR) technologies are improving light sensitivity and eliminating motion blur, enabling select cameras to overcome these typical challenges day and night.
With the latest low-light technology, cameras can capture license plate details even when light levels drop. The technology required to achieve this level of detail includes large pixel sensors, advanced image processing, and noise suppression algorithms to improve sensitivity. Combining these technologies provides more detail due to lower noise and boosts monochrome sensitivity to the next level to offer improved performance in extreme low-light scenes. Integrated or supplementary infrared illumination improves the ability of the camera to deliver images with readable license plates at night.
The latest advancements in HDR technology are also improving license plate capture in challenging lighting. Standard HDR technology combines multiple images with different exposure times to increase the overall dynamic range and capture details of images in scenes that have both light and dark areas. With the latest HDR technology, a combination of large pixel sensors, advanced optics, and imaging algorithms will generate two readouts from one short exposure with different gain levels.
This lens-sensor combination generates a high dynamic range frame without any HDR blur and artifacts. The result is an enhanced dynamic range (up to 144dB) that is motion-optimized to capture the highest level of detail in challenging scenes with both dark and bright areas, while eliminating blur and artifacts caused by moving vehicles.
Full Solution Considerations
After the camera captures the image, the LPR software will process the image for license plate detection. It will crop the image to focus on the license plate, process the cropped license plate image for optimal character finding and recognition, and capture the characters and recognize the license plate code for country or state.
Considerations for selecting the proper software include the performance of the LPR software in detecting the presence of a license plate, as well as the accuracy of the optical character recognition (OCR). While many LPR software providers will share data on OCR accuracy, it is only relevant if the software is first highly accurate at detecting the presence of a license plate in an image. If the software does not detect the presence of a plate with a high degree of accuracy, it decreases the overall capture rate and effectiveness of the solution.
Other features to review include the countries and state license plates supported by the software, and the ability to manage and match white, black or information lists with operator alerts based on these lists. The capability to conduct forensic searches of vehicles for post-analysis of events and the possibility to embed the software in other management systems for greater efficiency are also important.
Information useful to record in these solutions includes date and time of capture, direction of movement, plate number, and information on a match with the white, black, or information lists of predefined vehicles. With blacklists, operators can be notified immediately if a known license plate is detected by a camera. Once notified, the operator can trigger a nearby moving camera to track the vehicle and provide security personnel the visual information they need to respond quickly and accurately.
Integration with third-party information sources can also help to identify a stolen vehicle, insurance status, toll subscription status, or authorized taxi or service vehicle status. If a vehicle is identified as a potential risk, searches of the plate number help operators to see the different locations of the vehicle over time — providing an overview of how the vehicle moved through the area being monitored.
Tools to manage storage and deletion of video and plate data in compliance with local regulations is also important.
System Installation Tips
When installing a system, the correct mounting point and angle of the cameras are essential for capturing the best license plate images. Often, the LPR software supplier will specify maximum camera mounting angles — both horizontally and vertically. Installation with angles beyond these limits may reduce the reliability of the software. For best results, the horizontal angle should be as small as possible. While ensuring the vertical angle is within the LPR software supplier’s recommendations, it is best to mount the camera higher than the license plate to minimize headlight glare issues.
The software supplier will also specify the minimum number of pixels required, horizontally and vertically, for reliable decoding of the license plate number. This, combined with the resolution of the camera and the focal length of the lens determines the maximum distance at which the system can retrieve license plate numbers.
Lens quality is also important, especially in single camera, multilane applications. Placement of the camera should ensure enough contrast on the license plate, and lens distortion should be low enough to ensure enough pixels on target are captured for off-center lanes. IR lighting, required at night, should have an even light distribution pattern across all lanes.
It is important to also consider the maximum vehicle speed at which plates can be decoded for the application to ensure proper selection of cameras. Maximum vehicle speed determines the maximum camera shutter speed. For start-stop applications, an exposure time of 1/500 s is appropriate, whereas for speeds of 75 miles per hour, exposure times of 1/1750 s or 1/2500 s are more appropriate. The frame rate of the camera can be reduced at lower speeds or in start-stop applications, which in turn reduces the processing power required for the LPR software.
When choosing the LPR solution, confirm it can communicate with other systems being used for security, such as the video management system (VMS), if it isn’t already a native capability of the VMS. The VMS should be able to receive alarms from the LPR solution for real-time notifications of detected vehicles, ensuring operators only need to work within a single system — increasing efficiency.
Targeting Markets & Applications
There are a variety of applications for license plate capture and LPR solutions. In traditional security applications, LPR solutions are being used for access control, parking enforcement, law enforcement and for general security.
Using LPR for facility access control is one of the growth areas in this segment of the security industry. It involves monitoring which vehicles are entering an area and ensuring they are allowed to be there. These solutions help security personnel to control the vehicles that are allowed to enter a facility or to generally track entry and exits of vehicles. For automated control, they can be used to trigger gates to open or remain closed based on the license plate of the vehicle. They can also be used for real-time alerts of wanted plates.
At construction sites — where there is a high-rate of theft of copper and other materials — LPR solutions can be used to identify and alert to vehicles entering the area. This capability can help in reducing the occurrence of thefts and in the identification of thieves for prosecution.
In addition to security applications, this type of solution can be useful for managing logistics operations or for automating the routine tasks of vehicle and individual registrations at a location.
In parking enforcement, the solution can be used for automated payments from regular patrons of parking lots or garages. For example, a customer’s account could be charged for parking based on the system capturing the plate as the vehicle enters and exits the lot.
In transportation, LPR solutions can be used to automate and enforce tolling, helping to reduce traffic congestion and emissions at toll booths.
Other unique applications involve the use of LPR for business intelligence and marketing purposes. For example, one airport used information garnered from its LPR solution to attract airlines to invest in gate properties at its site.
There are a variety of applications ranging from low-speed environments, such as parking management and access control of vehicles, to high-speed settings, such as tolling and multilane enforcement applications on highways, that provide integrators with new opportunities for growth. Understanding the nuances of the hardware, software, and integration capabilities is essential to choosing the proper solution to meet the needs of the application.
Paul Garms is Director of Regional Marketing – Video Systems, North America, for Bosch Security and Safety Systems